U.S. Secretary of State John Kerry said Thursday his government is trying to assure South Korea and Japan of its unswerving commitment to extended nuclear deterrence so that the key allies won't be tempted to develop nuclear bombs themselves.

"We are working with Japan and the Republic of Korea (South Korea) in order to make sure they don't feel so threatened that they move towards nuclearization in self-help," the secretary said at a Senate hearing on the department's budget for fiscal year 2015.

His remarks came as concerns grow that Seoul and Tokyo, both already equipped with advanced civilian nuclear programs, may seek to go nuclear in a bid to counter security threats from nuclear-armed North Korea or China, which has been sharply increasing its military spending.

Kerry said with the proposed budget, the U.S. would bolster its "bedrock alliances" with South Korea and Japan.

His choice of wording may draw some media attention. The Obama administration often characterizes the alliance with South Korea as a "linchpin" of regional security and that with Japan as a "cornerstone."

The secretary also noted South Korea's agreement to increase its financial contribution to the stationing of 28,500 American forces on the peninsula.

Under the Special Measures Agreement signed in January, South Korea will pay 920 billion won ($870 million) in 2014 for the USFK presence costs, up 5.8 percent from last year's 869.5 billion won.

"Thanks to the State Department's work, South Koreans are now making the largest contribution they have ever made towards our joint security agreement," Kerry said.

On his trip to China last month, he said discussions focused on what more the Chinese can do to help denuclearize the peninsula.

At that time, the secretary said he and Chinese leaders exchanged some specific proposals on how to revive nuclear talks with Pyongyang.